Expects: AX    0014H
CX    event mask (events which you want sent to your handler)
bit 0 = mouse movement           (CX | 01H)
bit 1 = left button pressed      (CX | 02H)
bit 2 = left button released     (CX | 04H)
bit 3 = right button pressed     (CX | 08H)
bit 4 = right button released    (CX | 10H)
bit 5 = center button pressed    (CX | 20H)
bit 6 = center button released   (CX | 40H)
All events:      CX = 007fH
ES:DX address of your event handler code
Returns: CX    event mask of previous event handler
ES:DX address of the previously-installed event handler code
Info: This function works like INT 33H 000cH (which see for details).
The only difference is that upon return, you obtain the address
and event mask of the previously-installed event handler.

This provides a way to install an event handler temporarily; that
is, you can install one while performing a certain subroutine,
then restore the previous one when you exit that subroutine.

This also provides a way to chain event handlers.  Install a
handler for all events, and if you get an event which you don't
really care about, pass it on to the previously-installed handler
(assuming its event mask shows that it expects the event).

Notes: INT 33H 0018H provides a flexible means for installing up to
three specialized-event handlers.

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INT 33H 0014H: Exchange Mouse Event Handler